moment curvature analysis

huangyhg

moment curvature analysis
folks,
i need to do a moment curvature analysis on a tall column to assess the cracking in it and to apply proper crack factors. i am using sap2000 and etabs.
can anyone point me in the right direction or validate my assumptions.
1. generate the m-φ curve for the particular column based on geometry and amount of reinforcing for a range of axial loads.
2. run the analysis and look at the rotation of the column at the point of interest, by looking at the rotation about the axis of concern.
3. compare the moment at the particular load case you are interested in to the moment in the m-φ curve.
4. the ratio of the moments will give the cracking factor.
is the above procedure correct?
thanks
it will likely be an iterative process because i'm assuming that the initial run will be with a factor of 1.0.  because the section properties will vary along the length of the column, you can't really apply a cracked factor at a given point to the entire column.  
is there a reason you're doing a moment-curvature analysis for this?  i've typically only done a moment-curvature analysis when trying to use plastic design for concrete to ensure that the section where the hinge forms has enough rotational capacity to allow other hinges to form.
the reason i ask is that it sounds like you are trying to really hone in on a factor for cracking, but to do it right, you should be using a variable section (based on cracking) or just use the ieff equation out of aci.
i have a super column, the tripod column from my previous posts.
we are doing a wind tunnel analysis on the project and we want to ensure the building stiffness is accurately captured.
yes, i intend to vary the level of cracking at about 10' increments, since that's how i have broken up my column.
then you're procedure sounds reasonable, but it will be iterative.  when you do your first run you'll get some factors.  you'll have to run it again with those factors and pull off new factors.  you will have to continue to do this until it converges and the new factors are the same as the factors you input.
is this a truely critical issue for your strucutre?  
if so, then i might seperate the beam into 10 or 20 pieces and apply a different crack factor to each piece. the reason for this is that if you apply one moment of inertia to the column then you will still be over-estimating it is spots and under-estimating it in spots.  what you really want to do is capture the reduction in stiffness at the locations which crack the most.  
that level of detail may not often be justified. but, adjusting the moment of inertia of individual pieces will change the 2nd order / buckling behavior that you see during the analysis. so, if this is your primary reason for doing the analysis, it may be worth the extra effort.